1. Field of the Invention
The present invention relates to lithography, and in particular relates to systems and methods for performing single-radiation-pulse exposures in a manner that reduces image smear.
2. Description of the Prior Art
The process of manufacturing certain micro-devices such as semiconductor integrated circuits (ICs), liquid crystal displays, micro-electro-mechanical devices (MEMs), digital mirror devices (DMDs), silicon-strip detectors and the like involves the use of high-resolution lithography systems. In such systems, a patterned mask (i.e., a reticle) is illuminated with radiation (e.g., laser radiation or radiation from an arc lamp) that passes through an illumination system that achieves a high-degree of illumination uniformity over the illuminated portion of the mask. The portion of the radiation passing through the mask is collected by a projection lens, which has an image field (also referred to as a xe2x80x9clens fieldxe2x80x9d) of a given size. The projection lens images the mask pattern onto an image-workpiece to produce a pattern either in a photosensitive layer on the workpiece surface or directly on the workpiece because of a reaction between the workpiece surface and the incident radiation. The workpiece resides on a workpiece stage that moves the workpiece relative to the projection lens, so that the mask pattern is repeatedly formed on the workpiece over multiple xe2x80x9cexposure fields.xe2x80x9d
Lithography systems include an alignment system that precisely aligns the workpiece with respect to the projected image of the mask, thereby allowing the mask pattern to be precisely superimposed on previously exposed patterns. In most cases, the mask image needs to be precisely aligned to a pre-existing exposure field on the workpiece to provide the juxtaposed registration necessary to build up layers of the device being fabricated.
Presently, two types of lithography systems are used in manufacturing: step-and-repeat systems, or xe2x80x9csteppers,xe2x80x9d and step-and-scan systems, or xe2x80x9cscanners.xe2x80x9d With steppers, each exposure field on the workpiece is exposed in a single static exposure. With scanners, the workpiece is exposed by synchronously scanning the work piece and the mask across the lens image field. An exemplary scanning lithography system and method is described in U.S. Pat. No. 5,281,996. The projection lenses associated with steppers and scanners typically operate at 1X (i.e., unit magnification), or reduction magnifications of 4X or 5X (i.e., magnifications of xc2x11/4 and xc2x11/5, as is more commonly expressed in optics terminology).
The ability of a lithography system to resolve (or, more accurately, xe2x80x9cprintxe2x80x9d) features of a given size is a function of the exposure wavelength: the shorter the wavelength, the smaller the feature that can be printed or imaged. To keep pace with the continuously shrinking minimum feature size for many micro-devices (particularly for ICs), the exposure wavelength has been made shorter. Also, historically the device size has increased as well, so that the lens field size has steadily grown. The resolution of the lithography system also increases with the numerical aperture (NA) of the projection lens. Thus, in combination with reducing the exposure wavelength, the numerical apertures of projection lenses tend to be as large as can be practically designed, with the constraint that the depth of focus, which decreases as the square of the NA, be within practical limits.
A novel and unconventional lithography system that performs exposures using single pulses of radiation is described in U.S patent application Ser. No. 09/854,226, filed on May 10, 2001 by the present inventors and assigned to the same assignee and entitled xe2x80x9cLithography system and method for device manufacture,xe2x80x9d which application is incorporated herein by reference. This single pulse exposure system, referred to by the present assignee by the trademark CONTINUOUS LITHOGRAPHY(trademark) lithography system, has many advantages. These include providing a high throughput equal to or greater than the most advanced lithography scanners using a smaller-than-conventional exposure field size. In the CONTINUOUS LITHOGRAPHY(trademark) lithography system, the workpiece (wafer) moves continuously underneath the projection lens while exposure fields are formed on the workpiece with a single pulse of radiation. The temporal pulse length of the radiation pulses and the speed at which the workpiece moves is selected so that the exposure fields are imaged with a minimum of smearing of the mask image.
A preferred radiation source for the CONTINUOUS LITHOGRAPHY(trademark) lithography system is a pulsed laser. However, while pulsed lasers can provide radiation pulses that are short and intense, they are also relatively expensive. This adds to the cost of the overall lithography system. To reduce the cost of the lithography system, a flash-lamp radiation source can be used. However, the temporal pulse lengths of a flash lamp are in the millisecond to microsecond range, with greater pulse energies being available from longer pulses. Thus, the image smear (blur) caused by imaging a fixed reticle onto a rapidly moving workpiece can be appreciable with a flash lamp source.
Accordingly, it would be greatly advantageous to be able to use a high-energy, long-pulse-duration flash lamp or long-pulse-duration laser in the single-pulse CONTINUOUS LITHOGRAPHY(trademark) lithography system without experiencing the aforementioned image smearing.
The present invention relates to lithography, and in particular relates to systems and methods for performing single-radiation-pulse exposures in a manner that reduces image smear.
A first aspect of the invention is a mask holder system for oscillating a mask to provide motion compensation for performing single-pulse exposures of the mask onto a moving workpiece. The mask holder system includes a mask frame for supporting the mask. A drive unit is operatively connected to the mask frame for imparting an oscillatory motion to the mask frame that corresponds to the movement of the workpiece. The oscillation is coordinated with the single-pulse exposures so that the image of the mask and the workpiece move in the same direction at substantially the same speed during exposure.
An exemplary embodiment of the mask holder system utilizes parallel rails slidably connected to first and second sides of the mask frame to allow for the mask frame to move in a plane defined by the parallel rails.
A second aspect of the invention is a method of reducing image smear when forming an exposure field with a single pulse of radiation on a moving workpiece. The method includes oscillating a mask in a mask plane, and then illuminating a mask with a pulse of radiation while the mask moves in a direction such that an image of the mask moves in the same direction as the workpiece. The method further includes projecting an image of the mask onto the moving workpiece to form an exposure field. The method is repeated to form a plurality of adjacent exposure fields, with each exposure field formed from a single pulse of radiation. The radiation pulses may be from a flash-lamp or a pulsed laser (including a modulated continuous-wave laser); in either case, the radiation pulses have a temporal pulse length of about a microsecond or greater.
A third aspect of the invention is a lithography system for conducting single pulse exposures that includes a pulsed radiation source (i.e., a flash-lamp or pulsed laser), an illuminator for collecting the radiation pulses, the mask holder system as described above in connection with the first aspect of the invention, a projection lens and a workpiece stage. A mask is supported in the mask holder system. The motion of the workpiece via the workpiece stage, the oscillating motion of the mask, and the emission of pulses of radiation from the radiation source are coordinated by a main control unit so that the amount of image smear from imaging onto the moving workpiece is reduced.